1. 7 th SRFMW Close-out – Session 1: Drivers Medium term R&D for projects (NGLS,PX); Long term R&D for accelerators (i.e. what are the ultimate limits?) 1.3 GHz cavities of the Tesla geometry are the present technology benchmark for processing and testing, even as other projects now take up the activity in the U.S. – Infrastructure availability helps push the field – Materials science is needed to produce high Q at very high gradient – Target was 8e09 at 35 MV/m (200 mT surface field) 90% pass first test CW operation of cavities has emerged in many discussions – CW losses directly impacts cryogenic plant size and operational costs – Materials science is needed to produce very high Q at modest gradients ---- is materials R&D relevant to both Q and E at same time? – Targets: 4-6e10 at 18 MV/m at 2 K? 650- 1500 MHz All processes should produce high yield at the desired Q and E – First pass (gradient) yield is 80% at 20 MV/m for ILC, ~98% for JLab upgrade – While projects must plan and build only with what is well-developed, R&D must provide the improvements that build toward the next projects – Stewardship: cost, convenience, reliability… Dollars per MV in quantity, including rf and cryo systems

2. Session 2: Theory and basic measurements Closing of energy gap at high RF fields: – exp (-D/kT) can get large because D can vanish (BCS breaks down at high RF field)  serious theory needs to couple with the experiments, materials science – Not a strong limitation (on Q) for Nb3Sn – What about Niobium? Does it explain the HFQS? Can we come up with experiments to test this? – Model systems, model experiments – Isolate certain parameters Trapped vortices and hot spots: – How important? Contribution of total dissipated power of a cavity (impact on medium field Q0)? – Areas between hot spots is also important (power shifts there upon heating) – 1000 vortices = hot spot? (depends on thermal conductivity)

3. Session 3: raw material Cooley: Cold work  better furnaces Variability of cold work with local strain – recrystallization to get rid of dislocations Challenges – methods to repair / optimize lower quality cavities. (reduce variability) Ciovati: Data do not show need for high RRR or low Ta content to achieve good RF performance (Q or Eacc) Challenges – Highest Ta content tolerable: specification for Ta clustering? Possible action: make cavities out of different grades material using present (modern) facilities and techniques. – Need: what are the metrics for success? 2e10 at 18 MV/m… Dzyuba: Resistance as a means to measure H content shown; strained samples correlated with H uptake – H is higher with depth and strain. Challenges – unambiguous interpretation of such measurements in practical samples that include EP and heat treatments. --- cross comparison with multiple techniques on same samples Chandrasekaran: Thermal conductivity measured in single crystals and bicrystals; document effects of strain and H on phonon peak Phonon peak is regained with annealing above 800  C, but not consistently, depends on strain and crystal orientation. H retained after 1100  C + H after 800  C anneal? – THE PHONON PEAK CAN BE TURNED ON AND OFF!!!! LG amplifies… Challenges – identify deformation and heat conditions (crystal orientation/strain paths) that lead to retained dislocation content that suppresses phonon peak recovery. – Need thermal conductivity measurements perpendicular to the sheet / wall (recrystallized structures might divert heat flow, e.g. around small grain layers)

4. Wallace: Methodology for making large diameter ingots with preferred crystal orientation described. Challenges – practical development hurdles with expensive capital equipment discussed. DO WE NEED SINGLE CRYSTALS ANY MORE?? LG cavities are important… and control over the grain morphology is useful. TOWARD A COMPLETE DESCRIPTION OF MATERIAL DEFORMATION FROM UNDERSTANDING OF SLIP BEHAVIOR (See the aluminum beverage can industry) Kang: Surface damage identified with OIM/GROD maps – shows surface damage on order of 100 um. Single crystal deformation analyzed – role of preferred slip on {112} planes identified. In-situ deformation described. Challenges – Identify how dislocation substructure depends on crystal orientation and strain path. Mapar: Crystal plasticity finite element modeling motivations reviewed to support hydroforming, single crystal tubes. Challenges – develop material model that can adequately simulate measured stress-strain behavior with correct slip system activation. Kneisel: Large grain ingot slice fabricated cavity overview – generally higher Q 1400  C anneal effect of having increasing Q slope. Challenges – gaining acceptance of large grain as viable / competitive fabrication approach in community, pressure vessel codes (what softness can be tolerated?), weighing the pros and cons of 1400  C anneal. (session 3, cont.)

5. Session 4: Processing Advances Processing Questions “Processing” is what we do to the surface to manipulate it into what we want – chemical, mechanical, thermal What do we “want”? – “smooth” – “clean” – “dirty” – “Nb” – “safe” – “cheap” and “simple” – “reproducible” Delayed flux entry for high surface field operations. Low dissipation losses at useful operating fields.

6. Session 4 – Processing advances -- Suggestions: Pay more attention to thermal processing, there is more to baking than just liberating interstitial hydrogen. Be very aware of/control the gas environment during & following bake (passivation). Incremental HF is window on near-surface removal/refresh. >> Q 0 improvement opportunities – Careful material analysis is yet needed to develop an understanding of what is happening and then to manage it to realize best performing Q 0 at 2 K. We have dots without connection… Pay more attention to thermal cycle history, including cycles above T c and 100K. Flux expulsion mechanism appears to improve Q 0 at least under some circumstances. What is happening here? Process dynamics analysis in Nb EP have yet to fully incorporate gravitational self-diffusion effects into analysis of local reaction rates. To the extent that uniform polishing is important, this needs to be better understood and quantified.

7. (suggestions, cont.) CBP uniquely erases surface topography. Smoothness target justification is not yet clear, in general, for Nb cavity Q or field maximum. (Films need smooth surfaces to start) Be aware that hydride precipitates on Nb surface induce local plastic (i.e. semi-permanent) deformations that persist after subsequent dissolution. ACTION: low-temperature stages to watch this in-situ New detailed understanding is yet accessible for BCP effect on Nb surface topography. VEP is attractive for operational efficiency. Optimization of parameters for performance benefit has yet to be done. “Greener” processes are attractive and need yet to be fostered to maturity. Removing the need for HF chemistry would be great. Recognize that dealing with “defects” may require different strategies than “optimizing” the normal material. Robust process development must address both. (Prevent vs cure)

8. Session 5: Nb micro and nano Characterization What is the surface? – Chemistry (Elements, composition)) Lots of H (very mobile – even during measurement) – Structure (Orientation) Defects (GBs, dislocation density, pits, sensitive to processing) Growth of model Nb oxide structures Use of decomposition of the oxide (120 bake) – Localized measurements (PIT, hot/cold-spot sized areas) and global measurements (cavity Q) LDOS (EELS, gap, … ) --- how to make the local measurements relevant to the global performance? Is it representative - How many layers, how deep is your layer? – Cross-sections or depth profiling required Damage from beams, stirring up hydrogen, etc. Do cross-sectioning techniques, profiling influence composition (yes!) Is it representative - Handling – Is surface stable/protected between cavity/coupon test and measurement? Are the ex-situ, post-situ microscopic measurements representative of the cavity surface as it was tested.

9. Characterization tools Many new tools have been brought to bear! – New RF “microscopy” via recording head device – send samples! Especially those with previous measurements – New in-cavity laser interferometry Many topics addressed surface and near-surface structure and composition: – The surface itself – Chemical probes 10 to 100 nm deep – Structural probes 1 to 20 nm deep Using samples that were “hot” or “cold” to reveal what is important – A clean oxide (and a complete diffusion barrier) with a clean metal below? – An oxide with numerous defects… Magnetic defects “talk” to Cooper pairs Pathways through the diffusion barrier for H moving in or out – … a slightly dirty metal? Hydrogen: up to 40% just beneath oxide, possibly trapped on various sites Oxygen: up to 2000 ppm just beneath oxide, decays over 200 nm length Carbon, Nitrogen: segregation to boundaries and defects? – … a slightly dirty metal with defect sites and small precipitates? Hydrides… Others too? Proximity effect couples small metallic precipitates RF currents are induced across small precipitates – Strain and cold work vs annealed (lack of strain) and recrystallized / recovered samples – Does sample preparation leave behind a relevant specimen? Many specimens, broad sources, comparative measurements, contrasting conditions…

11. Session 6: Seamless Cavities Can we control thinning in spinning applications/ what is the limit of starting thickness on starting material? Is there a know/desired tube specification? Iteration is needed urgently. Should there be a collective focus on one tube size? A lot of progress in the 3.9 GHz tube size… Does this fit into funding and testing activities in a meaningful way? Explore… What tube size do we need/want? The tube size is driven partly by the forming process, in addition to the cavity frequency. System availability to hydroform tubes? Are there opportunities for generic materials work from tube R&D?